1. Field of the Invention
This invention relates to a photochemical film forming apparatus which forms a film on the surface of a substrate by radiating a light beam to photochemically dissociate a reactive atmosphere gas, and more particularly it relates to a photochemical film-forming apparatus adapted to deposit and grow a film of uniform thickness along the direction of travel of the light beam.
2. Description of the Prior Art
Film-forming techniques include a CVD (chemical vapor deposition) method based on plasma excitation. This plasma-excited CVD method uses a lower process temperature than that in the conventional CVD method and provides better step coverage than does the deposition based on physical methods such as evaporation and sputtering, these features accounting for its wide use. On the other hand, however, the plasma-excited CVD method has disadvantages such as ion-attack on the surface of the deposited film.
Thus, in recent years, a CVD method based on optical excitation (photo-induced CVD method) has been developed. This photo-induced CVD method operates on the principle of irradiating an atmosphere gas with a light beam to activate the atmosphere gas to accelerate its photochemical reaction, the resulting reaction product being deposited on the substrate.
FIG. 1 is a schematic view showing the relative disposition of a light beam and a substrate in a conventional photochemical film-forming apparatus using the photo-induced CVD method. In FIG. 1, a substrate 3 is held on a susceptor 4 to be exposed to an atmosphere gas. A deposition film will be formed on the main surface of the substrate 3. A light beam 1a for exciting the atmosphere gas is radiated from a light beam generator (not shown) and travels through an incident window 2 along a direction parallel to the main surface of the substrate 3. The distance between the light beam 1a and the substrate 3 is usually a few millimeters and typically 1-2 mm. The film-forming method will now be described.
The light beam 1a passes through the incident window 2 and irradiates the reactive gas of the atmosphere gas. Then, only the reactive gas molecules present in the path of travel of the light beam 1a absorb the light beam energy to be photochemically dissociated. The resulting reaction product from this photochemical reaction deposits on the surface of the substrate 3 immediately below the beam to form a film.
FIG. 2 is a graph showing how the rate of deposition of the film on the substrate changes when the arrangement shown in FIG. 1 is used. In FIG. 2, the horizontal axis indicates the distance measured in the direction of travel of the light beam, and the vertical axis indicates the rate of deposition of the film on the substrate. As can be seen from FIG. 2, the rate of deposition of the film on the substrate greately decreases along the direction of travel of the light beam. The reason is as follows.
The reactive gas (atmosphere gas) absorbs the incident light beam energy and is photochemically dissociated in accordance with the intensity of the incident light beam. On the other hand, however, as a result of the absorption of the light beam by the reactive atmosphere gas, the intensity of the light beam exponentially decreases as the light beam travels, according to the Lambert-Beer law expressed by the relation, I(x)=I.sub.0 exp(-x/.alpha.), where I.sub.0 is the initial intensity of the light, .alpha. is the absorption factor, and x is a distance along the travelling direction of the light. With this decrease in the light beam intensity, the extent to which the photochemical reaction is induced decreases, so that the amount of reaction product decreases and so does the rate of deposition.
A photochemical film-forming apparatus and film forming method of prior art is disclosed in P. K. Boyer et al., "Laser-Induced Chemical Vapor Deposition of SiO.sub.2 ", Appl. Phys. Lett. 40 (8), 15 Apr. 1982, pp. 716-718.
However, the aforesaid prior art has given no consideration to the uniformity of the rate of deposition of the film on the substrate.